The present invention relates generally to improvements in methods and apparatus for the batch digesting of cellulosic material such as wood chips, and more particularly to a process and apparatus for reducing the amount of equipment necessary for the construction of displacement batch digester cooking systems.
In conventional batch processes for digesting wood chips, the digester is filled with chips before being charged with the cooking liquor which typically includes sodium hydroxide and, in a kraft process, further includes a sulfur compound. The digester is then sealed and, with steam, the temperature of the digester is brought up to a cooking temperature at which the digester is maintained for a period of time referred to as "the cook". At the conclusion of the cook, a blow valve in the digester is opened, and the contents of the digester are then discharged into a blow tank by virtue of the hot liquor in the digester flashing into steam and forcing the delignified pulp out of the digester.
In the conventional batch process described above, much of the heat energy acquired by the contents of the digester during the process exits through the blow tank with the exhaust vapors. To date, the most successful attempts at recovering this heat energy include displacement heating systems which are disclosed in U.S. Pat. Nos. 4,578,149, 4,601,787, 5,059,284 and 5,080,757, all of which are commonly assigned to Beloit Corporation of Beloit, Wis. The systems disclosed in these patents utilize what is commonly referred to as liquid displacement technology and the systems themselves are commonly referred to as displacement batch digester cooking systems.
In a displacement batch digester cooking system, at the end of a cook, the digester is held under pressure and a displacement liquid is used to displace the hot black or spent liquors under pressure resulting in a displacement of the hot black liquor out of the digester at the cooking temperature. After being displaced from the digester, the hot black liquor is collected in a high temperature accumulator. The typical displacement fluid is filtrate obtained from washing the pulp or delignified fibers obtained in the earlier digestion of chips in other batches. The filtrate has a temperature elevated from the usual room or ambient temperature because it has already been passed through warm, delignified fibers. Thus, while the filtrate has a temperature of less than the hot black or spent liquor, it may still have a temperature of about 70.degree. C.
In the digester, the filtrate will absorb heat from the solids in the digester and its temperature will increase. Accordingly, a remaining portion of the hot spent liquor and a portion of the now-heated filtrate contain a sufficient amount of heat that is worth saving. Thus, the remaining hot black liquor and a portion of filtrate are stored in a low temperature accumulator. The rest of the filtrate is used in draining the digester and removing the delignified pulp.
Then, to begin a new batch, the digester is filled with new wood chips. To utilize the energy retained in the low temperature accumulator, the low temperature mixture which comprises a portion of the hot spent black liquor from the previous batch and filtrate used to displace the hot spent black liquor is transferred from the low temperature accumulator to the digester. As a result, the chips are heated.
After treatment with the fluid from the low temperature accumulator, liquor from the high temperature accumulator is pumped into the digester to displace the fluid from the low temperature accumulator. The fluid from the high temperature accumulator consists mostly of the hot spent black liquor from the previous batch. As a result of being immersed in the liquid from the high temperature accumulator, the temperature of the wood chips or solids is raised again.
As a final displacement process, a portion of the fluid from the high temperature accumulator is displaced by a mixture of hot black liquor from the high temperature accumulator and hot white liquor from a hot white liquor accumulator. The resulting mixture is used as the cooking liquid. After the cook, the cycle begins again with the displacement of the spent cooking fluid with washer filtrate.
In prior art systems using the above-described general process, separate booster pumps are required to transfer fluid from the low temperature and high temperature accumulators and the digester. Further, a booster pump is required to transfer hot white liquor from the hot white liquor accumulator and the digester. An example of the complexity and number of booster pumps required is illustrated in FIG. 1.
Specifically, a system 10 is illustrated which includes a digester 11 and communication by way of a series of valves and conduits to a filtrate reservoir or tank 12, a low temperature accumulator 13, a pair of high temperature accumulators 14, 15, a hot white liquor accumulator 16 and a cool tank 17. As discussed above, filtrate from the filtrate reservoir 12 is used to displace spent hot black liquor from the digester 10 after the cook. Thus, filtrate from the filtrate reservoir 12 is pumped toward the digester 11 by way of a conduit 17, a filtrate displacement pump 18, a conduit 19, through a valve 20, a valve 22, a conduit 23 and into a lower end 24 of the digester 11.
Powered by the displacement pump 18, the filtrate displaces the hot spent liquor out an upper end 25 of the digester 11 by way of a conduit 26, valves 27 and 28, a conduit 31 and valves 32 and 33 before entering high temperature accumulators 14 and 15 respectively. For purposes of clarity, the high temperature accumulator 15 is intended to receive the first portion of the hot spent liquor after the cook and therefore has the highest temperature. Accordingly, the hot spent liquor from the cook enters the accumulator 15 through the valve 33 first while the valve 32 is closed, thereby isolating the high temperature accumulator 14 from the conduit 31 temporarily. Then, as the accumulator 15 reaches capacity, the valve 33 is closed and additional hot spent liquor proceeds through the valve 32 into the high temperature accumulator shown at 14. Circumstances may also arise where it is necessary to permit the hot liquor to flow through a valve 30 and into the low temperature accumulator 13.
A heater shown at 33 may be used to heat fluid in the high temperature accumulator 14 as it is circulated through a pump 34 and a valve 35. A valve 36 permits a make-up of fluid from the high temperature accumulator 14 into the high temperature accumulator 15. Further, an overflow of fluid from the high temperature accumulator 14 may flow through a conduit 37, a heat exchanger 38 and a conduit 39 into the low temperature accumulator 13. Similarly, overflow from the low temperature accumulator 13 passes through a trim cooler 41, through a valve 42 and into the cool tank 17. Overflow from the cool tank 17 is pumped by a pump 43 into a weak liquor storage tank and evaporation area (not shown).
After the spent liquor has been displaced with the filtrate, a preliminary displacement of filtrate with cold fluid from the tank 17 is pumped by a cool pad pump 44, through conduits 45 and 23 into the lower end 24 of the digester 11. The purpose of the cool pad displacement is to prevent flashing of any low temperature liquor from the low temperature accumulator 13 during the subsequent displacement. Filtrate displaced by the cool liquid during the cool pad displacement is returned through a valve 46, a conduit 47 and a valve 48 to the cool tank 17. The remaining filtrate and cool pad fluid are then displaced by liquid from the low temperature accumulator 13 which is pumped by a warm fill pump 51, through a conduit 52, valves 53 and 22 and the conduit 23 into the lower end 24 of the digester 11.
The typical temperature of the fluid from the low temperature accumulator 13 is about 125.degree.C. This fluid is then displaced by fluid collectively pumped from accumulators 14 and 15. The typical temperature of fluid in the high temperature accumulator 14 is about 150.degree.C. while a typical temperature for the high temperature accumulator 15 is about 165.degree. C. Appropriate amounts are then drawn from each accumulator 14, 15 and pumped by a hot fill pump 54, through a valve 55, the valve 22 and the conduit 23 and into the lower end 24 of the digester 11. Displaced fluid from the low temperature accumulator 13 flows primarily through the return line 47 to the cool tank 17.
However, depending upon the condition of the low temperature fluid and other process variables such as the type of chips being digested, at least a portion of the low temperature liquor may be returned to the low temperature accumulator 13. After the low temperature liquor from the accumulator 13 has been displaced by liquor from the high temperature accumulators 14 and 15, at least a portion of the high temperature liquor is displaced by fresh hot white liquor from the accumulator 16.
The hot white liquor flows through a valve 56 and is pumped by the hot fill pump 54 into the lower end 24 of the digester. As hot white liquor is injected into the lower end 24 of the digester 11, excess hot liquor flows out the upper end 25 of the digester through the conduit 26, the valve 27, a valve 59 and into a conduit 60. Depending upon the temperature of the hot liquor, it will flow through the valve 61 into the low temperature accumulator 13 or through a valve 49 into the high temperature accumulator 14. A heater may be provided at 57 for the purpose of heating cool white liquor retrieved from a reservoir shown at 58 which has passed through the heat exchanger 38 to cool fluid flowing from the high temperature accumulator 14 to the low temperature accumulator 13. Cool white liquor also passes through a valve 81 before it is mixed with low temperature liquor in a conduit 82 during the warm fill and cool white liquor is also passed through a valve 83 before it is mixed with cool liquid in a conduit 84 during the cool pad procedure.
At this point, the digester is full of fresh hot white liquor from the accumulator 16 and hot liquor from the accumulators 14, 15 as well as the wood chips to be cooked. The cooking process is begun and, to increase the temperature inside the digester 11, a recirculation pump 61 is employed which draws fluid out of a recirculation conduit 62 disposed near the middle of the digester 11 and pumps the fluid through a digester heater 63 (which is typically heated with steam (not shown)) before the fluid passes through a valve 64 and reenters the top of the digester 11 through the conduit 26 and valve 65 and reenters the bottom 24 of the digester 11 through the conduit 23. During recirculation, the valves 27 is closed.
After the cook is completed, the hot liquor disposed in the digester 11 is displaced using filtrate from the reservoir 12 as discussed above. The filtrate is removed using a pump-out pump 66 which pumps the filtrate and pulp through the valve 67 and to a discharge tank (not shown). Additional fluid from the filtrate reservoir 12 is pumped by a dilution pump 75, through a conduit 76 and a valve 77 into the digester 11 which serves to assist in the flushing of the remaining filtrate and pulp out of the digester and to further cool the digester 11. During the subsequent recharge of the digester 11 with wood chips and during the initial injection of cool pad liquid from cool tank 17, air is permitted to escape through an air evacuation valve shown at 68. The wood chips are further initially packed with steam through a valve shown at 69. A relief valve is shown at 71 and additional condensate is discharged through a valve shown at 72.
As shown in FIG. 1, a large number of pumps are employed. Specifically, the hot fill pump 54 is needed between the high temperature accumulators 14, 15 and hot white liquor accumulator 16 and the digester. The warm fill pump 51 is needed between the low temperature accumulator 13 and the digester 11. Further, the separate cool pad pump 44 and dilution pump 18 are needed between the cool tank 17 and filtrate tank 12 and the digester 11 respectively.
Further, the separate dilution pump 75 is required between the filtrate reservoir 12 and the digester 11. The large number of pumps required adds to the capital cost and operating cost of the system. Further, in a typical system, ten digesters are operated simultaneously and therefore the large number of pumps required for each digester contributes greatly to the high capital costs and operating costs required to run these systems.
Accordingly, there is a need for an improved displacement batch digester cooking system which simply uses less equipment and, more specifically, fewer pumps and associated headers and instrumentation. Still further, the operation of a separate washer filtrate reservoir 12 and cool tank 17 with their associated pumps is costly both in terms of to capital and operating costs as well as space requirements. Accordingly, there is a need for an improved displacement batch digester system which incorporates fewer tanks as well as less pumps.